1. Field of the Invention
The present invention relates to an ion microscope adapted for imaging the fine structure of complex objects, such as macromolecules and microorganisms. The ion microscope is adapted whereby the surface structure of an object can be observed at high resolution, and whereby internal structure of an object can be observed by combining the controlled removal of surface material with sequential surface imaging.
2. Description of the Related Art
Microscopes which are now available are limited in their ability to image the fine structure of complex objects. In the light microscope, this limit exists at about 200 nm and is imposed mainly by the diffraction of light. In the transmission electron microscope, the diffraction limitation is less severe because of the much shorter wavelength of the electron beam. This permits much higher resolution to be obtained.
A number of other factors are involved in limiting resolution. In the case of the transmission electron microscope, it is difficult to observe structure that is much finer than the thickness of the object. Very thin sections are used and these are supported on very thin films to minimize this limitation. There is also the disruption of fine structure that is caused by electron beam bombardment, and by the processing needed to prepare the object for imaging. When observing thin films of inorganic materials, resolutions at or near atomic structure can be obtained. When observing a small organic object, such as a ribosome having a size of 15 by 10 nm, a resolution of 3 nm is typically the best that can be done. The transmission electron microscope operating under optimum conditions is capable of showing little more than the general shape of such an object. The detailed structure involving hundreds of thousands of atoms cannot be imaged in this way.
The scanning electron microscope provides an image of an object's surface by scanning an electron beam over its surface in a systematic raster pattern while detecting the secondary electrons which are emitted from the object. With organic objects, this emission is from a relatively large region that relates to the penetration and spreading of the scanning beam. The size of this region, in combination with lens aberrations and diffraction effects, limits the fineness of detail which can be observed to less than that of the transmission electron microscope.
The atomic force microscope uses a very sharp electrode tip to scan in a raster pattern over the surface of the object. Forces relating to the structure of the object are sensed and displayed. Resolution capability relate to the fineness of the tip and the way it interacts with the object. At the present time, there are considerable difficulties in imaging the fine structure of organic objects using this microscope.
There are several special purpose microscopes which relate in part to the microscope of the present invention. The first is the point projection field ion microscope described by E. W. Muller, Zeitschrift Physik, vol. 131, page 136, in which the well rounded tip of an electrode is imaged at very high resolution by virtue of the linear projection of ions from the tips surface to a fluorescent viewing screen. The ions are produced by a very intense electric field at the tip's surface which causes the disassociation of gas atoms or molecules which condense on the tip's surface. Imaging is limited to materials which can stand the high forces associated with the intense fields needed to produce the ions. The second is the point projection ion microscope described in the Colterjohn Jr., U.S. Pat. No. 2,548,870 in which ions for imaging are generated in an ion source and directed in a parallel beam at the sharply rounded tip end of an electrode, upon which objects to be imaged are mounted. The ions are repelled after coming close enough to the surface to be subject to localized deflection and neutralization. The ions after repulsion are divergent and project an image of the tip's surface upon a fluorescent screen or photographic film. This design has the disadvantage of having a very small field of view because the transverse velocity of the ions at the tip's surface increases quickly as a function of off axis position. This increases the interaction region of an ion with the surface and results in loss of resolution. This design also has the disadvantage of not having a lens aperture. This causes a lack of sensitivity to small deflections produced by object induced field distortions.
The microscope of the present invention is most closely related to the design presented in the Colterjohn patent. It differs from this patent in a number of ways, including provision of mechanisms whereby the severe limitation on the field of view is overcome and imaging sensitivity is improved.